The present invention relates to a method and/or architecture for charge pumps generally and, more particularly, to a method and/or architecture for low voltage supply high efficiency cross-coupled high voltage charge pumps.
Charge pumps are circuits that pump charge into capacitors to develop an output voltage higher than the supply voltage. High voltage charge pumps provide positive or negative high voltage to program/erase programmable elements such as EEPROM and flash memory, power solid-state particle detectors and photo-multipliers, drive analog switches, etc. Multiple charge pump circuits can be implemented serially to increase the voltages provided. Conventional charge pump circuits include a number of serially connected stages. The stages contain a diode (or transistor configured as a diode) and a capacitor. The stages are driven by a clock signal.
Referring to FIG. 1a, a diagram of a circuit 10 illustrating a conventional charge pump is shown. The circuit 10 illustrates a so-called Dickson charge pump circuit (see J. F. Dickson, xe2x80x9cOn-Chip High-Voltage Generation in MNOS Integrated Circuits Using an Improved Voltage Multiplier Techniquexe2x80x9d, IEEE J. of Solid-state Cir., vol SC-11, No. 3, June, 1976, pp. 374-378, which is hereby incorporated by reference in its entirety). The circuit 10 includes a number of stages 12a-12n. Each stage 12 contains a diode connected transistor 14 and a capacitor 16.
The circuit 10 can provide a supply voltage minus threshold voltage (Vccxe2x88x92Vt) increase at each stage. The output voltage Vpp of the circuit 10 can be Vpp=(Vccxe2x88x92Vt)*n+Vcc. However, the amplitude of clock pulses xcfx86a and xcfx86b, pump capacitance (Cp), stage parasitic capacitance (Cs), and load current (Io) are factors that can limit the voltage gain achieved at each pump stage.
For charge to be passed fully from a lower stage to a higher stage, the increase in voltage for the stage xcex94Vstage must be greater than the transistor threshold voltage Vt. The circuit 10 has a number of disadvantages. For example, (i) the diode drop reduces efficiency, (ii) the breakdown voltage of the transistors must increase as charge pump output voltage increases, (iii) extra stages are required due to low efficiency, (iv) an output voltage at least one Vt above the desired output voltage must be developed, and (v) the circuit 10 does not work well at low supply voltage levels.
Referring to FIG. 1b, a diagram of a circuit 20 illustrating another conventional charge pump is shown. A description of the circuit 20 can be found in Jieh-Tsong Wu and Kuen-Long Chang, xe2x80x9cMOS Charge Pumps for Low-Voltage Operationxe2x80x9d, IEEE J. of Solid-state Cir., Vol. 33, No. 4, April, 1998, pp 592-597, which is hereby incorporated by reference in its entirety. The circuit 20 is implemented similarly to the circuit 10 of FIG. 1a. However, to improve low supply voltage performance, each stage 22 of the circuit 20 has an additional transistor 28. The circuit 20 is more efficient than the circuit 10 and eliminates the voltage threshold Vt drop. However, the circuit 20 has disadvantages in that (i) the charge can flow backwards when the clock signals xcfx86a and xcfx86b transition low at each corresponding stage and (ii) the circuit 20 must develop an output voltage that is at least one Vt higher than the required output voltage Vpp.
Referring to FIG. 1c, a diagram of a circuit 30 illustrating another conventional charge pump circuit is shown. A description of the circuit 30 can be found in Jieh-Tsong Wu and Kuen-Long Chang, xe2x80x9cLow Supply Voltage MOS Charge Pumpsxe2x80x9d, 1997 Symposium on VLSI Circuits Digest of Technical Papers, pp 81-82, which is hereby incorporated by reference in its entirety. In order to eliminate the charge backflow problem of the circuit 20 of FIG. 1b, a charge transfer switch circuit 40 is added at each stage 32a-32(nxe2x88x921). The circuit 30 has an advantage when compared to the circuit 20 that charge is always pumped forward. However, the circuit 30 has the disadvantages of (i) an output voltage that must be at least one Vt above the desired output voltage and (ii) the transistors and capacitors at the stages 32nxe2x88x921 and 32n must have a high breakdown voltage (breakdown voltage=Vpp+Vt where Vt can be as high as 3V).
It would be desirable to have a charge pump circuit that can efficiently develop a high positive or negative output voltage from a low input voltage without requiring higher breakdown voltage transistors.
The present invention concerns an apparatus comprising a number of cross-coupled charge pump stages configured to generate an output voltage in response to (i) a supply voltage, (ii) a first signal, and (iii) a second signal, where the output voltage has a greater magnitude than the supply voltage.
The objects, features and advantages of the present invention include providing a method and/or architecture for implementing a low voltage supply higher efficiency cross-coupled high voltage charge pump that may (i) have improved efficiency, (ii) have reduced diode drop, (iii) perform well at low supply voltages, (iv) reduce the need for higher breakdown voltage transistors, (v) require little extra die area, and/or (vi) be easily implemented.